~/.xmonad: make firefox "official".

i've been using it for months now, so why not make it easier to spawn?

.xmonad/xmonad.hs

        layoutHook = avoidStruts $ smartBorders $ layoutHook defaultConfig,
        handleEventHook = fullscreenEventHook
     } `additionalKeys` [
        ((mod4Mask, xK_b), spawnHere "chromium --allow-file-access-from-files"),
        ((mod4Mask .|. shiftMask, xK_b), spawnHere "chromium --incognito --allow-file-access-from-files"),
        ((mod4Mask, xK_b), spawnHere "firefox"),
        ((mod4Mask .|. shiftMask, xK_b), spawnHere "firefox -private-window"),
        ((0, xF86XK_AudioMute), spawn "amixer set Master toggle"),
        ((0, xF86XK_AudioLowerVolume), spawn "amixer set Master 5%-"),